Flash memory has advantages such as having a small cell size, a large memory capacity and a high collision resistance. Thus, in recent years, the demand for flash memory has rapidly increased. A typical flash memory includes a control gate electrode and a floating gate electrode that make a pair. Electrons are injected into the floating gate electrode when data is being written, while electrons are discharged from the floating gate electrode when data is being erased. Such flash memory further includes an insulation film covering the floating gate electrode. The insulation film prevents the leakage of electrons injected into the floating gate electrode. Thus, the flash memory is capable of storing data for a long time insofar as electrical power is not supplied thereto.
If the insulation film covering the floating gate electrode, for example a tunnel insulation film, an interlayer capacitance film having an ONO (Oxide-Nitride-Oxide) structure, etc. is thickened, it is difficult for electrons to leak from the floating gate electrode and a data holding characteristic of a flash memory can be thereby enhanced. Thickening the insulation film covering the floating gate electrode, however, leads to a fall in efficiency of injecting and discharging of electrons into and from the floating gate electrode, thereby deteriorating data writing characteristics and data erasing characteristics.
Further, when the control gate electrode is formed by etching during the manufacture of flash memory, side walls of the floating gate electrode, which is composed of multicrystllaine silicon, are exposed. For this reason, a process for covering the side walls with an oxide film by heat treatment is performed. However, the tunnel insulation film or the interlayer capacitance film having an ONO structure is oxidized and thickened due to such heat treatment, thereby causing a problem where the efficiency of injecting and discharging electrons into and from the floating gate electrode deteriorates. In order to solve the aforementioned problem, a technique has been suggested wherein a silicon oxide film is formed on a surface of the floating gate electrode by ion-implanting nitrogen to the floating gate electrode after forming the floating gate electrode and then heat treating at a temperature of 800° C. or more, thereby segregating nitrogen at an interface between the floating gate electrode and the interlayer capacitance film and the tunnel insulation film (that is, a silicon nitride film is formed). (see Patent Document 1)
Patent Document 1: Japanese Patent No. 3312102
When forming a silicon nitride film by ion-implanting nitrogen to the floating gate electrode as suggested in the above-mentioned Patent Document 1, it is possible to form a silicon nitride film wherein nitrogen is segregated only in the interface between the floating gate electrode and the insulation film. However, in the technique suggested in the above-mentioned Patent Document 1, heat treatment after implanting nitrogen is indispensable. Accordingly, the aforementioned heat treatment is disadvantageous in that the thermal budget increases and the distribution of implanted nitrogen is difficult to precisely control.
Meanwhile, it is known in the art to use plasma nitridation as a means of forming a silicon nitride film by thinly introducing nitrogen to a surface of a silicon layer. This may be applied to nitridation of the floating gate electrode. The plasma nitridation is advantageous in that the nitrogen distribution is easily controlled when compared to the method of heat treating after implanting nitrogen as suggested in the above Patent Document 1. Further, use of plasma allows for nitridation at a low temperature of, for example, 400° C.˜600° C., thereby eliminating all problems caused by heat treatment at a high temperature of 800° C. or more, which is suggested in the above Patent Document 1. However, selective nitridation is difficult in the plasma nitriding process. That is, since all exposed surfaces of a target object to be processed are nitrided, a part not needing to be nitrided is also formed with a nitrogen-containing layer. If an unnecessary nitrogen-containing layer remains in the flash memory, it may lead to degraded performance of the flash memory, for example, the occurrence of electrical interference between neighboring cells via the remaining nitrogen-containing layer.
For example, when forming a silicon nitride film on a surface of a floating gate electrode of a flash memory, if the plasma nitridation is performed after forming a floating gate electrode on a silicon substrate, not only is the surface of the floating gate electrode nitrided, but the element separation film separating neighboring cells are also nitrided and a silicon oxynitride film is formed thereon. As a result, an originally unnecessary nitrogen-containing layer (silicon oxynitride film) remains on the element separation film of the finished flash memory. This remaining unnecessary silicon oxynitride film may cause electrical interference between neighboring cells to thereby deteriorate a data holding characteristic of the flash memory.